1. Field of the Invention
The present invention relates to optical switching devices and finds particular application in high bit rate communications links.
2. Related Art
Communications links which can carry high data rates are advantageous because they can transmit increased levels of information and/or can provide links via a single physical connection which service higher numbers of customers. For instance, time division multiplexed signals on a communications link can provide a higher number of time slots, and potentially therefore service a higher number of customers, where the link itself carries a higher data rate.
In order to access the data on a communications link, it is necessary to down load the information on the link to a receiver. In time division multiplexing, one particular customer will require information from only one or more selected time slots to be down loaded. To do this, switching devices may be used, the speed of switching of the device being commensurate with the capability of the link for carrying high speed traffic. In optical communications, it is envisaged that data rates may be achieved as high as 100G bits/s in the foreseeable future. This might be carried in ten time slots, providing ten channels at 10G bits/s.
As well as potentially increasing the number of customers using a communications link, ultra high bit rate links in future networks may enable customers to be offered new large bandwidth services as well as giving telecommunications companies greater flexibility in managing their networks. The present invention is concerned with an important part of achieving ultra high bit rate links, providing a type of switch capable of demultiplexing a 10G bit/s bit stream from a 100G bit/s optical time division multiplexed (OTDM) signal. An optical clock signal might be used to switch out every tenth bit. Hence, to recover all the data on the link, ten switching devices might be used in parallel.
Switching devices are known, including switching devices which can be controlled by an optical input. For instance, in "130 ps" Recovery of All-optical Switching in a GaAs Multi Quantum Well Directional Coupler" published in Applied Physics Letters volume 58 number 19 on 13th May 1991 by Li Kam Wa et al (Ref. (i)), a recovery time of 130 ps is reported in a zero gap directional coupler using multiple quantum wells. However, it has now been discovered in making the present invention that much faster recovery times can be achieved. Reduced recovery times provide potentially faster switches, recovery time being a limiting factor.